This invention relates generally to a solution and a method for coating aluminum and aluminum alloy surfaces. More particularly, this invention relates to a solution and a method for the conversion coating of aluminum and aluminum alloy surfaces as well as the unique coated surfaces produced thereby to prevent corrosion and to improve adhesion to over-lying paints, inks, lacquers and plastic coatings.
Aluminum and aluminum alloy conversion coatings are typically applied to sheet and roll stock which is formed into a wide variety of different shapes for applications ranging from architectural to canning. In all of these applications, it is important that the conversion coatings be thin and even, and capable of very rapid application. These coatings should also adhere well to both the underlying metal and the overlying coating.
The conversion coatings must be capable of protecting the underlying aluminum from attack by corrosive agents in the environment including water and water vapor. The coating should also be sufficiently flexible and elastic to enable the coated metal to be formed into the desired shapes without cracking the conversion coating and adversely affecting both the appearance of the overlying coating and the corrosion resistance of the overall product.
In many applications, such as in the coating of aluminum cans, it is important that the conversion coatings be clear and colorless in order to preserve the natural appearance of the aluminum. Coated aluminum cans must also have the ability to resist discoloration when they are heated during processing of the cans at temperatures up to 175.degree. F.
While coating compositions and methods are known for producing aluminum and aluminum alloy conversion coatings having the required and desired properties, the use of these compositions and methods or the nature of the conversion coatings which they produce have had shortcomings. For example, until recently the most popular and effective materials used for producing aluminum and aluminum alloy conversion coatings contained chromates and/or ferricyanides, both of which are highly toxic and therefore extremely difficult to handle and environmentally very undesirable. Alternative coating compositions, while generally free of the handling and environmental difficulties, produce conversion coatings which are poorly adherent, porous or deficient in some other respect. As a result, there has been a very significant impetus to those working in this art to develop coating compositions and methods which are safe to use, yet produce coatings equal to or better than those formerly obtained with chromates or ferricyanides.
For example, U.S. Pat. No. 3,682,713 describes a process for treating aluminum and other metals with a chromate free solution containing fluorides of boron, titanium, zirconium or iron, free fluoride ions and oxidizing agents. These treating solutions optionally also include metal salts which form insoluble oxides of the coating such as the water-soluble salts of zinc, nickel, cobalt and trivalent chromium.
Although this patent generally describes a pH range of 3.0-6.8 and a treatment time range of 3 to 180 seconds, the examples of the patent reflect a working minimum pH of 4.3 and a working minimum treatment time of 20 seconds, which respectively adversely affect the corrosion resistance of the conversion coating produced and slow the overall coating process.
U.S. Pat. No. 3,964,936 which issued to one of the present inventors, reflects yet another chromate ferricyanide free coating solution. This solution contains zirconium, a fluoride source, and, optionally, boron and specifically excludes manganese, cobalt, iron and nickel. The coating solution of this patent is designed to operate in a pH range of about 3 to about 5 and at a minimum contact time of about 15 seconds.
U.S. Pat. No. 4,148,670 describes yet another aluminum coating solution. This solution contains zirconium and/or titanium, fluoride and phosphate. As in the case of the '936 patent referred to above, this patent stresses that the claimed solution is capable of effectively forming coatings on aluminum in the absence of elements such as manganese, iron, cobalt, nickel, etc. Also, while this patent describes a pH range of about 1.5 to about 4.0 and preferably about 2.6 to about 3.1 and a contact time of at least about 5 seconds and preferably 15 seconds, the examples illustrate a working pH of at least 2.5 and a contact time of about 25 seconds.
U.S. Pat. No. 4,273,592, which issued to the same inventor and assignee as the above '670 patent, contains as essential ingredients a zirconium and/or a hafnium compound, a fluoride compound and a polyhydroxy compound having no more than seven carbon atoms. The pH of this coating solution is about 3.0 to about 5.0 and preferably about 3.0 to about 4.0. However, this patent goes on to teach that a preferred treatment solution will have a pH in the range of about 3.0 to about 4.0, and the examples utilize a minimum pH of 3.5. Also, this patent specifically requires that the coating solution be free of boron and phosphate.
Furthermore, the above solution requires 20 seconds' contact time to produce the desired coating. It is therefore not capable of forming effective coatings in the less than five seconds' contact time permitted in many modern rapid coating systems. For example, this solution could not be used in high speed coil lines which may permit only five seconds' contact time.
Finally, U.S. Pat. No. 4,277,292 discloses a conversion coating solution utilizing zirconium and fluoride ions in combination with tannin and operating at a pH between 2.3 and 2.95. This patent explains that its low pH bath is preferable to prior titanium/fluoride/tannin baths because it avoids acid depletion and precipitation. However, unfortunately, this patent claims that the coating solution requires 30 seconds' of contact time to produce the desired coating. Therefore, as in the above '592 patent, this solution is not capable of forming effective conversion coatings in the much shorter contact times permitted in many systems. In addition, it should be noted that the solutions of this patent exclude the use of phosphates.
While the conversion coating solutions and methods of the above patents offer useful alternatives to the prior chromate and ferricyanide based systems, they leave much to be desired. For example, the zirconium fluoride complexes produced by the solutions of these prior patents often exhibit inferior adherence characteristics in the conversion coating. In addition, these prior solutions often require excessive coating reaction times which slow down automated aluminum processing lines. Also, in systems such as that described in the above '670 patent, the levels of zirconium and fluoride ions must be maintained in a very delicate balance in order to avoid precipitation of insoluble zirconium phosphate, which would deplete the coating solution of two of its required components. Finally, all of the above prior arts do not actually provide secondary cleaning or removal of smut products caused by the alkaline cleaning operation often necessary on high speed coil lines to achieve superior adhesion characteristics.
The coating solution and method of the present invention utilize zirconium and fluoride in combination with certain other ions including nickel to produce a conversion coating with outstanding corrosion resistance and adhesion. This coating can be produced extremely rapidly, and without sacrificing the required clarity, colorlessness, resistance to darkening when subjected to boiling water, and elasticity. The coating solution of the present invention is also capable of providing secondary cleaning action.